1. Field of the Invention
The present invention relates to a carbon dioxide concentration sensor that is sterilizable with steam under pressure at a temperature ranging from 120 to 130 degrees Celsius, and a method of using such sensor.
2. Prior Art
The importance of measurement of dissolved oxygen was suggested by Hixon, Gaden and Finn in the 1950s in the fermentation engineering field. As a result, a dissolved oxygen sensor was developed and has been widely used which can withstand sterilization with steam under pressure. It has recently been found that control of a fermentation process can advantageously be effected by measuring carbon dioxide as well as dissolved oxygen in the fermentation broth or gas discharged from the fermentor.
In the medical field, a sensor for electrochemically measuring the concentration of carbon dioxide in blood was already developed by Stow [Arch. Phys. Med. Rehabil., 38,646 (1957)] and Severinghaus [(J. Appl. Physiol., 13,515 (1958): Ann. N.Y. Acad. Sci., 148,115 (1965)], and has been in widespread use as a Pco.sub.2 probe. Such sensor comprises a rod-like combination-type pH electrode including a reference electrode therein and covered by a membrane which is highly permeable to carbon dioxide gas, there being a thin layer of aqueous solution containing alkali bicarbonate interposed between the membrane and a glass surface serving as a sensing element of the pH electrode. When the sensor is placed in a liquid or gas containing carbon dioxide, CO.sub.2 gas permeates the membrane and gets disolved in an electrolyte solution in the sensor, wherein part of the CO.sub.2 gas produces carbonic acid and bicarbonic acid, thereby changing the value of pH of the electrolyte solution.
It has already been attempted to use this type of sensor in the fermentation engineering field. However, the coventional Pco.sub.2 sensor has not found wide usage because when it is sterilized with steam under pressure, the electrolyte solution boils, the membrane is damaged, and the pH electrode loses its sensitivity due to heat.
The present inventors have provided a sensor and a method of using the same which are suitable especially for use in the fermentation engineering field, based on improvement of a sensor of the type proposed by Stow and Severinghaus.